A highly monochromatic high-power 10-W-class visible light source is advantageous for realizing a large-size image display apparatus, a high-brightness display apparatus and the like. To meet the demand for such image display apparatuses, laser diodes have been adopted to realize high-power light sources for red and blue light sources among the three primary colors of red, green and blue. However, for the green light source, a high-power semiconductor laser light source has not yet been in practical use, and instead a wavelength conversion element, that converts a fundamental wave emitted from a solid state laser medium into a second harmonic wave, has been adopted to realize a high-output green laser light.
For such high-output green laser light source, a wavelength conversion device capable of converting the wavelength of the fundamental wave with high efficiency is particularly demanded. To meet this, various proposals have been made. For example, the structure, which realizes improved conversion efficiency, by lengthening an optical waveguide using a return optical waveguide provided in the wavelength conversion element (see, for example, Patent Document 1). Also for the structure adopting a solid laser with laser diode excitation, the structure wherein an optical path is lengthened by causing multiple reflections of a laser beam of the solid laser inside the wavelength conversion element has been proposed to realize the wavelength conversion with high efficiency (see, for example, Patent Document 2).
Another structure has been proposed wherein the fundamental wave that has entered the wavelength conversion element and has transmitted without being converted into a second harmonic wave are caused to be reflected from a reflector, and the fundamental wave reflected from the reflector are caused to enter the wavelength conversion element again in different optical path. This structure realizes a high output power by repeating the foregoing process (see, for example, Patent Document 3). With this structure, a high-output inexpensive wavelength conversion element which can convert the wavelength with high efficiency can be realized with a simple structure.
Another proposal has been made to add a feature of collecting the incident light by a light collecting optical system to be focused onto an intermediate point of an optical path formed between both ends of a wavelength conversion element and controlling the light collecting conditions to reduce the volume of the incident signal light beam to the minimum (see, for example, Patent Document 4). Furthermore, an optical system having two wavelength conversion elements optically connected in series with a lens provided in-between has also been proposed. With this structure, it is possible to realize a high-output laser light source which enables still improved wavelength conversion efficiency with simpler structure.
Incidentally, in the wavelength conversion element, generally it can be said that the smaller is the beam diameter of the fundamental wave, the higher is the wavelength conversion efficiency. However, according to the foregoing conventional structure, the fundamental wave is collected only once in the wavelength conversion element. Therefore, although modifications have been made to increase the wavelength conversion efficiency with the state collected once, it is still difficult to obtain overall high conversion efficiency. Moreover, when the intensity of light of the fundamental wave is high and the intensity of light of the second harmonic wave as generated is also high, a significant amount of heat is generated by absorbing light inside the wavelength conversion element. Therefore, a mismatch occurs with the phase matching temperature in one stage and other stages subsequent to that stage, and quality of light in that stage deteriorates. The resultant problem is that the wavelength conversion efficiency is lowered in one stage and stages subsequent to that stage, thereby making it difficult to realize overall high output power of the second harmonic wave and an overall improvement in the wavelength conversion efficiency.    Patent Document 1: Japanese Patent Application Laid-open No. S60-57825    Patent Document 2: Japanese Patent Application Laid-open No. H2-185081    Patent Document 3: Japanese Patent Application Laid-open No. 2006-208629    Patent Document 4: Japanese Patent Application Laid-open No. 2007-58191